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I've seen angry used with either "at" or "about". Below you will find my interpretation. Could you please decide if I'm right or not?

angry at
The teacher was angry at Nell.--> The teacher was angry at one particular moment. E.g. Nell didn't bring in her homework and the teacher told her off.

angry about
The teacher was angry about Nell. --> The teacher was angry because of Nell. E.g. Nell fell in love and wasn't preparing for her classes etc. The teacher's angriness may have lasted for a certain (longer) period of time.

Please list more examples of correct usage patterns.

Thanks in advance, as always.
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It isn't to do with timescale. It is about who or what is on the receiving end of the anger.

Angry at Nell - the teacher was angry directly with Nell - Nell was there when the teacher reacted angrily towards her.

Angry about Nell - the teacher was angry regarding something to do with Nell but Nell wasn't there.
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PastsimplePlease list more examples of correct usage patterns.

Search yourself for them in the online literature at:

with:
site:literaturepost.com "angry about"
or
site:literaturepost.com "angry at"
or, combining the above:
site:literaturepost.com "angry (about OR at)"
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Comments  
PastsimpleI've seen angry used with either "at" or "about". Below you will find my interpretation. Could you please decide if I'm right or not?

angry at
The teacher was angry at Nell.--> The teacher was angry at one particular moment. E.g. Nell didn't bring in her homework and the teacher told her off.

angry about
The teacher was angry about Nell. --> The teacher was angry because of Nell. E.g. Nell fell in love and wasn't preparing for her classes etc. The teacher's angriness may have lasted for a certain (longer) period of time.

Please list more examples of correct usage patterns.

Thanks in advance, as always.
A lot of the time, people say "angry at" SOMEONE, but "angry about" a SITUATION. I think you have picked up that idea. But it's possible for people to stay angry (or mad) at others for a long time.

Examples:

I'm angry at myself right now. (I didn't do nearly as much work as I should have this week.)

I'm angry about American politics.

I'm angry about his decision.
Nona The BritIt isn't to do with timescale. It is about who or what is on the receiving end of the anger.

Angry at Nell - the teacher was angry directly with Nell - Nell was there when the teacher reacted angrily towards her.

Angry about Nell - the teacher was angry regarding something to do with Nell but Nell wasn't there.
OK, I made a mistake with the duration Emotion: embarrassed but I hope I was close with my descriptions:

The teacher was angry at Nell.--> Nell didn't bring in her homework and the teacher told her off.
The teacher was angry about Nell. --> The teacher was angry because of Nell. E.g. Nell fell in love and wasn't preparing for her classes etc.

However, they're no match for you accurate description.

Nona and Nef, thank you very much!
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 Marius Hancu's reply was promoted to an answer.